Grafted polyolefins are well known and can be prepared by a variety of methods. Commonly, the polyolefin is treated with peroxides and then brought into contact with suitable monomers. As a result of the treatment with the peroxides, which act as initiators, free radicals are first formed at the polymer chain, with which monomers can undergo an addition reaction in a grafting reaction. Heat, radiation, and other techniques can also be used to graft monomers to polyolefins. The grafting can be done in a variety of ways. One method, illustrated in U.S. Pat. Nos. 3,862,265, 3,953,655 and 4,001,172, grafts monomers such as maleic anhydride to polypropylene in an extruder. U.S. Pat. No. 4,537,836 grafts monomers to polyethylene in a Brabender mixing unit. U.S. Pat. Nos. 5,344,888 and 5,367,022 melt graft monomers to polyolefins such as propylene and ethylene-propylene copolymers. U.S. Pat. No. 5,140,074 heats the polyolefin in a reactor under nitrogen and introduces a free-radical initiator and a vinyl monomer. U.S. Pat. No. 5,411,994 irradiates a high-porosity particulate polyolefin and thereafter adds a liquid vinyl monomer. U.S. Pat. Nos. 5,128,410 and 5,229,456 prepare graft copolymers by dissolving or swelling a polyolefin in an inert solvent, heating and stirring the mixture while adding a grafting monomer and a radical initiator, followed by volatilizing the solvent, for example in a devolatilizing extruder.
Grafted polyolefins are especially useful as blend components and can impart adhesion to the blend. For example, U.S. Pat. Nos. 4,774,144, 4,842,947, 5,336,721, and 5,439,974 describe several blends employing graft polyolefins.
Despite the many methods for the preparation of grafted polyolefins, all methods can suffer by having some unreacted grafting monomer or other low molecular weight species as impurities in the grafted polyolefin. These impurities can have a deleterious effect on certain properties such as color, adhesion, or film clarity.
U.S. Pat. No. 4,698,395 purifies the grafted polyolefin by dissolving the grafted polyolefin in an organic solvent and then precipitating particles with aqueous base. This process can be difficult if starting with solid grafted polyolefin. For instance, in Example 3, they purify 100 parts of grafted polyolefin with 2,000 parts of xylene and 2,000 parts of aqueous sodium hydroxide. Such large volumes are costly and the process requires dissolving the grafted polyolefin.
Japanese Pat. No. 2185505 immerses molten grafted polyolefin in a carbonyl containing solvent such as methyl ethyl ketone to remove impurities. This process requires melting the grafted polyolefin and it can be difficult to isolate the purified product.
Japanese Pat. No. 4202202 purifies grafted polyolefin by refluxing with a mixture of toluene and methyl ethyl ketone, followed by rinsing with acetone. Toluene and methyl ethyl ketone do not form an azeotrope. This process uses three solvents and recovery of solvents is difficult, thereby adding to the cost and complexity of the process.
There remains a need for a simple, efficient, and low cost process to remove impurities from grafted polyolefins. The impurities can have a deleterious effect on certain properties such as color, adhesion, or film clarity.